1. Field of the Invention
The present invention relates to a magnetic bearing system for use in a turbo-molecular pump, a machine tool or the like.
2. Description of the Related Art
A magnetic bearing system is often used in equipment requiring high-speed rotation, such as a turbo-molecular pump or a machine tool. The magnetic bearing system is designed to detect a displacement of a rotor using a displacement sensor, and feed back the detection result to control an excitation current of an electromagnet. Specifically, the displacement sensor is typically composed of an inductance type which is operable to amplitude-modulate a carrier applied thereto, depending on an impedance change resulting from a change in a gap relative to the rotor. After A/D converting the amplitude-modulated signal, the digital signal is demodulated, and the demodulated displacement signal is used for controlling the excitation current of the electromagnet (see, for example, JP 2004-144291A).
The modulated signal to be subjected to the A/D conversion has a bandwidth ranging from (fc−fr) to (fc+fr), wherein fc is a frequency of the carrier, and fr is a frequency (maximum frequency) of a displacement signal of the rotor. That is, the bandwidth (maximum frequency) of the modulated signal is 2 fr. Generally, in an operation of A/D converting an analog signal at a sampling frequency fs, it is necessary to satisfy a relation of 4 fr<fs according to the sampling theorem to prevent the occurrence of aliasing, and therefore the maximum frequency fr of the rotor displacement signal is limited to fs/4. Thus, depending on a processing speed of digital signal processor (DSP), the maximum frequency fr cannot be set at a desirably high value, resulting in poor feedback characteristics in a high frequency range.
Specifically, while it is conceivable to increase the sampling frequency fs and/or the carrier frequency as measures for avoiding the problem about deterioration in the high-frequency characteristics, these measures will inevitably impose higher processing load on a DSP, and lead to the need for a DSP operating at high frequencies, causing an increase in cost. Moreover, if the digital demodulation is performed in a conventional manner, an S/N ratio will deteriorate due to signal decay occurring during the demodulation operation, which is likely to cause vibration in the rotor which is an object to be supported.